The imbibition printing dye transfer process is well known. According to common procedures, a tanned colloid relief image is formed by imagewise exposure of a suitable light sensitive layer on a support, differentially hardening the colloid layer in accordance with the imagewise exposure, and removing the colloid from the support in inverse proportion to the amount of imagewise light exposure. The differential colloid hardening and removal is conventionally performed with a pyrogallol hardening developer as described, e.g., in U.S. Pat. No. 2,837,430, the disclosure of which is hereby incorporated by reference. For full color prints, three separate relief images corresponding to the blue, green, and red color records of the image being reproduced may be formed in separate blue, green, and red light sensitive matrix films by respective exposures with blue (approx. 400-500 nm), green (approx. 500-600 nm), and red (approx. 600-750 nm) light though a color negative film. The resultant colloid relief images are then dyed with yellow, magenta and cyan dyes, and the dye images transferred to a mordant-containing receiver film. In this manner imbibition printed colored dye images may be obtained which faithfully reproduce a colored subject.
The imbibition process normally results in a sensitometric Density vs. Log-Exposure curve shape with a relatively sharp (high contrast) "toe", or lower scale, region for the developed matrix films and resulting imbibition prints. The toe region is generally regarded as the curved region below the straight, or mid-scale, region of a D-LogE sensitometric curve. Reducing the toe area contrast, or "softening" the toe, is desirable to extend the latitude of the matrix film. One process which may be used to control the toe contrast is "flashing". Flashing is the non-selective low level exposure of a photographic material with the intent of softening the toe region of the sensitometric curve. While flashing of photographic materials to control contrast is a well known procedure, imbibition printing matrix films are unique in that the light sensitive layer of the matrix film generally has a large portion of a visible light absorbing non-photosensitive material, such as carbon particles, coated along with silver halide and colloid materials. The carbon absorbs light as it passes through the matrix film, thus concentrating the exposure towards the base (the exposure in this process is conventionally made through the base). A normal flash exposure with this type of material accordingly will not control the curve shape in the desired manner to the desired extent.
In years past, green and red matrix films having sufficient native blue sensitivity have been flashed with blue light in order to control the lower-scale sensitometry. A yellow dye was added to the matrix film which had the effect of lowering the contrast of the flash exposure. This allowed good control of the curve shape. The blue matrix film, however, could not use the yellow dye for the flash exposure control since the main image exposure is also made with blue light which would be absorbed by the yellow dye. Thus, the blue matrix film was flashed with blue light without the presence of a yellow dye.
Problem to be Solved by the Invention
In years past, coarser (larger grain size) emulsions were used with inherent lower toe contrasts. The blue matrix film (very coarse grain emulsion) was low enough in contrast that a minimal blue flash was required to control toe contrast. For blue matrix films made with modern fine grained emulsions, however, which are inherently relatively higher in contrast, there is not an effective blue flash toe contrast control. Additionally, when using green and red matrix films containing excess yellow absorber dye, there is a tendency for the film to become very brittle resulting in cracking and degradation of the dye image, as well as dirt generation in manufacture and use of the film. While the lower inherent contrast of previously used coarser emulsions required only relatively low levels of yellow absorber dye in the green and red matrix films for sufficient toe contrast control, modern fine grain emulsions used in the green and red matrix films are also inherently relatively higher in contrast and much larger quantities of the yellow absorber dye is needed to control the toe contrast. This can lead to physical problems such as tackiness, brittleness and film fracturing in manufacture of the film. It would be desirable to provide effective toe contrast control for each of the blue,.green and red imbibition printing matrix films without such physical problems, and especially to provide such control in a consistent manner.